In conventional communication systems, a coded video bit stream may be transmitted over a heterogeneous wired-to-wireless network. For example, the coded video bit stream may be transmitted from a wired Internet connection to a wireless mobile receiver. Due to differences in loss conditions and bandwidth constraints between the wired and wireless portions of the network, it is common in such coded video bit stream transmissions to partially or fully decode the coded video bit stream, reencode the video at an appropriate rate, and then add error resiliency properties prior to transmission to the wireless receiver. An example of a technique of this type is described in G. de los Reyes et al., “Video Transmission For Resilience in Wireless Channels,” IEEE International Conference on Image Processing,” Chicago, Ill., pp. 338-342, October 1998.
Other known video bit stream adaptation techniques have focused primarily on scalability of the video bit stream to different bandwidth conditions. For example, layered video coders have been developed which generate video bit streams consisting of several layers, usually a base layer followed by one or more enhancement layers, as described in S. McCanne, “Scalable Compression and Transmission of Internet Multicast Video,” Ph.D. Thesis, University of California Berkeley, December 1996. The layered video coder allows users with greater bandwidth capability to subscribe to more layers, and thereby receive better video quality, while users with less bandwidth capability can subscribe to the base layer only. This provides adaptive video quality for different users while minimizing the bandwidth inefficiency associated with transmission of the video at several different bit rates.
Current video coding standards such as MPEG-2 and H.263 also have layered coding profiles, with options of temporal, spatial, signal-to-noise ratio (SNR) and hybrid scalability. For example, the MPEG-2 standard describes a spatially-scalable profile which consists of two layers, a digital standard television resolution layer and an enhancement layer for high definition television. See, e.g., B. Haskell et al., “Digital Video: An Introduction to MPEG-2,” Chapman and Hall, 1997. However, the scalability options for these current standards come at the expense of compression performance. These and other layered coders offer scalability on a coarse level in that each layer must be completely received in order to be utilized in enhancing the video quality.
Transcoders have also been used to provide bit rate adaptability in applications such as the above-noted wired-to-wireless heterogeneous network. Conventional transcoding techniques generally focus on either bit rate conversion or conversion between standards, e.g., H.263 to H.261 conversion. Since the current video coding standards generally have the same basic components, transcoders are designed to reuse as many of the components as possible. For example, it is highly desirable to reuse the motion vector estimates, since their generation is a computationally very expensive process. Other techniques have addressed the issue of reducing the overall bit rate, e.g., converting from a high bit rate studio format to a low bit rate distribution format, while utilizing as much of the original bit stream information as possible, e.g., the original motion vector estimates and coding mode choices.
A significant problem with conventional transcoders is that decoding and recoding the video bit stream can adversely impact the resulting reconstructed video quality. In addition, the decoding and recoding operations can introduce substantial additional delay as well as computational complexity in a video transmission system.
Known video transmission techniques for lossy channel conditions have focused on either adding error resilient features to well-defined coding standards such as H.263++ and MPEG-4, or on channel coding for either packet losses over Internet protocol (IP) connections or wireless channel conditions. Examples of the former types of techniques are described in J. Wen et al., “A Class of Reversible Variable Length Codes For Robust Image and Video Coding,” Proceedings of the IEEE International Conference on Image Processing, Santa Barbara, Calif., Vol. 2, pp. 65-68, October 1997; J. Villasenor et al., “Robust Video Coding Algorithms and Systems,” Proceedings of the IEEE, Special Issue on Video Transmission For Mobil Multimedia Applications, Vol. 87. No. 10, pp. 1724-1733. October 1999; and N. Fäber et al., “Extensions of ITU-T Recommendation H.324 For Error-Resilient Video Transmission,” IEEE Communications Magazine, pp. 120-128, June 1998, while examples of the latter types of techniques are described in C. Leicher, “Hierarchical Encoding of MPEG Sequences Using Priority Encoding Transmission (PET),” Berkeley Technical Report TR-94-058, November 1994; B. Girod et al., “Packet Loss Resilient Internet Video Streaming,” Visual Communications and Image Processing '99, SPIE, San Jose, Calif., January 1999; A. E. Mohr et al., “Graceful Degradation Over Packet Erasure Channels Through Forward Error Correction,” Proceedings of the 1999 Data Compression Conference (DCC), Snowbird, Utah, pp.1-10, 1999; K. W. Stuhlmüller et al., “Scalable Internet Video Streaming With Unequal Error Protection,” Packet Video Workshop, New York, pp. 1-8. April 1999; R. Puri et al., “Multiple Description Source Coding Using Forward Error Correction (FEC) Codes” 33rd ASILOMAR Conference on Signals, Systems, and Computers, Pacific Grove, Calif., IEEE, pp. 342-346, 1999; B. Girod et al., “Feedback-Based Error Control for Mobile Video Transmission,” Proceedings of the IEEE, Special Issue on Video Transmission For Mobile Multimedia Applications, Vol. 87, No. 10, pp. 1707-1723, October 1999; and P. G. Sherwood et al., “Error Protection For Progressive Image Transmission Over Memoryless and Fading Channels,” IEEE Transactions on Communications, Vol. 46, No. 12, pp. 1555-1559, December 1998.
Unfortunately, the above-noted conventional techniques have been unable to provide an efficient general framework for video transmission over a heterogeneous network, which allows bit rate scalability, adaptability across different network conditions, and graceful degradation in the presence of channel errors. A need therefore exists in the art for such a framework.